In many cases, there is a need to synchronize timing signals maintained in network elements of a data transfer network in such a way that the timing phases of the timing signals are equal to each other with a sufficient accuracy. For example, in mobile networks of the new generation, a prerequisite for reliable data transfer to and from a mobile terminal device moving from a service area of one base station to a service area of another base station is that the difference between timing phases of the timing signals of these base stations is at most 1 microsecond. Each timing signal comprises typically a train of timing points pitched at substantially uniform temporal intervals. The timing points can be e.g. rising or descending edges of pulses or zero-crossings of a waveform, etc. Thus, in the above-mentioned mobile networks, the temporal difference between mutually corresponding timing points of the timing signals maintained in the base stations should be at most the above-mentioned 1 microsecond.
In one prior art solution, network elements, e.g. base stations, which need to maintain mutually synchronized timing signals are provided with radio receivers for receiving timing information from one or more satellites of a satellite system. The received timing information enables each network element to maintain a timing signal which is synchronous with corresponding timing signals prevailing in the other network elements. The satellite system can be for example the US Global Positioning System “GPS”, the Russian GLONASS, the European Galileo system, or the Chinese Beidou system. A network element may, however, become incapable of utilizing the timing information transmitted by the satellite system because of e.g. radio jamming and/or some other reasons. Therefore, there is a need for a backup system for generating a sufficiently accurate secondary timing signal during time intervals when the network element is incapable of utilizing the timing information transmitted by the satellite system.
The specification 1588-2008 of the Institute of Electrical and Electronics Engineers “IEEE 1588-2008” defines a protocol for maintaining mutually synchronized timing signals in network elements of a data transfer network. In accordance with the IEEE 1588-2008 protocol, the network elements send timestamp messages to each other. Based on the timestamp messages each network element controls the timing signal it maintains. The calculation of the difference between the timing phases of timing signals maintained in two network elements is based on an assumption that data transfer times are equal in both directions between the network elements under consideration. In packet-, frame-, and cell-switched data transfer networks, the above-mentioned assumption related to the transfer times does not often hold true with a sufficient accuracy because the data transfer time has a remarkable random-type portion and, in addition, data transfer paths between two network elements, routed to opposite transfer directions, may have different lengths and/or numbers of hops. The above-mentioned random-type portion is due to for example changes in routes configured in the data transfer network and queuing delays subjected to data packets, frames, or cells in transmission buffers and/or reception buffers of the network elements. Due to the above-mentioned reasons, the IEEE 1588-2008 protocol is typically not suitable for generating a sufficiently accurate secondary timing signal when the network element is incapable of utilizing the timing information transmitted by the satellite system.
The specification ITU-T G.8275.1 of the International Telecommunication Union “G.8275.1” defines a protocol for maintaining mutually phase-synchronized timing signals in network elements of a data transfer network. The G.8275.1 protocol is based on the IEEE 1588-2008 protocol, the main difference being in that the timestamp messages are terminated at the receiving end of each Ethernet link between the network elements, and the timing frequency and phase are regenerated at each intermediate network element. Hence, the phase inaccuracy due to indeterminate queuing delays and asymmetric data paths is eliminated. The G.8275.1 protocol, however, requires that each network element is able to operate according the G.8275.1 protocol making it impossible to utilize the protocol in existing networks with older network elements.